Automatic power management in optical communications systems in general, and Automatic optical Power Reduction (APR) in particular, is a challenge that others have attempted to overcome with varying degrees of success. The need for automatic optical power reduction arises primarily due to safety considerations. In particular, intensity of light exiting a severed optical fiber can easily damage eyesight of individuals not wearing eye protection.
Two competing considerations, speed and reliability, complicate the challenge of providing automatic power reduction. On one hand, optical power needs to be automatically reduced in less than one second after a fiber has been severed in order to maximize safety. On the other hand, optical power must not be automatically reduced needlessly, as with false detection of severed fiber.
Past attempts to address the challenges associated with speedy and reliable automatic power reduction have taken two forms: telemetry-based solutions and reflection-based solutions. Telemetry-based and reflection-based power reduction schemes exhibit varying levels of reliability for signaling LOS under different operating conditions. These solutions also incorporate response mechanisms of varying speed and reliability according to various situations.
Telemetry-based solutions have generally relied on detecting Loss Of Signal (LOS) at a network element that is downstream from an upstream emitting light source, and then communicating an LOS indicator upstream via an Optical Supervisory Channel (OSC). The upstream emitting light source undergoes automatic power reduction in response to the LOS indicator. Telemetry-based solutions have been implemented in various ways, and at least two embodiments are available depending on whether fiber is unidirectional or bidirectional.
Referring to FIG. 1, a first telemetry-based embodiment generally for use with two unidirectional fibers relies on detecting LOS at a downstream network element 20 resulting from fiber discontinuity 22, 24, and/or 26 on a first fiber. The LOS may be detected in the OSC at an out-of-path, downstream detector inherent to OSC circuit pack 28, or in the actual data channel at in-path, downstream detector 30 of downstream Erbium-Doped Fiber Amplifier (EDFA) circuit pack 32. If the LOS is detected at the in-path, downstream detector 30, a signal from downstream EDFA circuit pack 32 to OSC circuit pack 28 indicates LOS. In either case, an LOS indicator is then communicated upstream to upstream network element 34 via an OSC over a second fiber, and the indicator is received at OSC circuit pack 36. OSC circuit pack 36 then signals the LOS indicator to EDFA circuit pack 38, and the corresponding upstream emitting light source undergoes automatic power reduction.
There exist several disadvantages with regard to this first telemetry-based embodiment. For example, LOS may only be detected in the OSC for fiber discontinuities in a portion of fiber shared by the OSC and the actual data channel. Thus, LOS from fiber discontinuity 22, 24, and/or 26 may be detected at in-path, downstream detector 30, but the out-of-path, downstream detector inherent to OSC circuit pack 28 cannot detect LOS from fiber discontinuity 22 and/or 26. Also, a scheme that relies on detecting LOS at the out-of-path, downstream detector inherent to OSC circuit pack 28 cannot function properly if OSC circuit pack 28 becomes defective or is pulled. Similarly, if OSC circuit pack 36 becomes defective or is pulled, and/or a fiber discontinuity exists in the second fiber, then automatic power reduction for the corresponding EDFA of EDFA circuit pack 38 becomes unavailable. Further, where Raman pump 40 is collocated at downstream network element 20 and providing optical gain to the first fiber by transmitting optical power in the upstream direction, Raleigh backscattering along the first fiber can, in some circumstances, overpower the actual data signal and mask LOS due to fiber discontinuities such as 22 and 24, and cause in-path, downstream detector 30 to fail to detect LOS. This disadvantage further renders the first telemetry-based embodiment unsuitable for triggering automatic power reduction for Raman pump 40, which should reduce power in the event of a fiber discontinuity at 22, 24, 26, and/or 42.
Referring to FIG. 2, a second telemetry-based embodiment generally for use with a single bidirectional fiber relies on detecting LOS relating to a counter-propagating OSC signal from a downstream network element 20 at the upstream network element 34. According to this embodiment, a first OSC signal 44 constantly propagates from OSC circuit pack 28 to OSC circuit pack 36 along the bidirectional fiber, and a second OSC signal 46 constantly propagates from OSC circuit pack 36 to OSC circuit pack 28. If the first OSC signal 44 is no longer detected at OSC circuit pack 36, then OSC circuit pack 36 communicates an LOS indicator signal to EDFA circuit pack 38, which performs automatic power reduction in response.
There also exist several disadvantages with regard to this second telemetry-based embodiment. For example, LOS may only be detected in the OSC for fiber discontinuities in a portion of fiber shared by the OSC and the actual data channel. Thus, LOS from fiber discontinuity 24 may be detected, but this power reduction scheme cannot detect LOS from fiber discontinuity 22 26, 26, and/or 42. Also, a scheme that relies on detecting LOS via a counter-propagating OSC signal 44 cannot function properly if OSC circuit pack 28 becomes defective or is pulled. In such a case, LOS may be falsely detected at OSC circuit pack 36 and cause unnecessary automatic power reduction for critical traffic due to a failure of non-critical traffic. Similarly, if OSC circuit pack 36 becomes defective or is pulled, then automatic power reduction for the corresponding EDFA of EDFA circuit pack 38 becomes unavailable.
For further understanding of telemetry-based solutions, reference may be had to the following patents: U.S. Pat. No. 5,615,033, entitled Optical Signal Transmission Apparatus and Method, issued to Yoshida et al.; U.S. Pat. No. 5,914,794, entitled Method of and Apparatus for Detecting and Reporting Faults in an All-Optical Communications System, issued to Fee et al.; U.S. Pat. No. 5,943,146, entitled Optical Transmission System in which No Arrival of a First Light Signal is Notified from a First Station to a Second Station by an Alarm Light Signal Multiplexed with a Second Light Signal in Wavelength, issued to Harano et al.; U.S. Pat. No. 6,194,706 B1, entitled Methods and Systems for Locating Buried Optical Cables, issued to Ressl; U.S. Pat. No. 6,344,915 B1, entitled System and Method for Shutting Off an Optical Energy Source in a Communication System Having Optical Amplifiers, issued to Alexander et al.; and U.S. Pat. No. 6,359,708 B1, entitled Optical Transmission Line Automatic Power Reduction System, issued to Goel et al.
In contrast with telemetry-based solutions, reflection-based solutions have generally relied on detecting a downstream LOS using principles of Raleigh backscattering and/or Fresnel reflection. Thus, an in-path detector is typically placed at an upstream network element to detect back-scattered and/or reflected light resulting from a down stream fiber discontinuity. An optical power threshold is set and the upstream emitting light source undergoes automatic power reduction in response to the detected back-scattered and/or reflected light. Reflection-based solutions have been implemented in a number of different ways, and at least two embodiments are available.
Referring to FIG. 3, a first reflection-based embodiment shows two types of reflection-based detectors. For example, a back-reflection optical detector 48 detects light that is back reflected from downstream using principles of Raleigh backscattering and/or Fresnel reflection. Also, an optical tap 50 downstream of EDFA circuit pack 52 feeds optical power back to tap-based optical detector 54. These optical detectors are more reliable than those of telemetry-based solutions in one sense because they reside on the same circuit packs as the emitting light sources that need to undergo APR in the event of a downstream LOS. Communication of detected LOS to the light source in question is thus greatly simplified, but these detectors also have some disadvantages.
There are several inherent shortcomings of reflection-based detection mechanisms for APR. For example, the amount of reflected power generated at a fiber discontinuity is dependent on the characteristics of the endface at the location of the fiber break, which is unpredictable. Thus, if the Fresnel reflection off the fiber end-face is weak as a result of a highly uneven surface, the power levels detected back at the emission source will not be high enough to trigger a shut down of the source. As a result, LOS may not be detected despite the fact that unsafe power levels may still be exiting off the broken fiber. Also, the amount of back-reflected power reaching the emitting source is dependent on the distance between the source and the location of the fiber break, which is also unpredictable. Thus, optical detector 48 may detect a fiber discontinuity at 22, but not at 24 or 26 due to the greater distance, especially with Ultra Long Haul (ULH) systems, and the discontinuity at 22 may also go undetected in some situations depending on the end-face characteristics of the fiber break. Further, the above shortcomings are also true of discontinuities in the fiber path caused by the removal of optical connectors, especially angle-polished connectors (APC), which are designed to minimize back-reflections. Still further, tap-based detector 54 cannot detect a discontinuity that extends beyond the optical tap. Moreover, with Raman pump 40 flooding the fiber with optical power, optical detector 48 can be overwhelmed if a low threshold is set, so that a higher threshold must generally be used in the presence of a downstream Raman pump. Thus, the problem is further compounded if a fiber discontinuity at 42 lowers optical power received by optical detector 48, such that a fiber break at 22 may not even suffice to raise optical power received by optical detector 48 above the high threshold. As a result, the fiber breaks at 22 and 42 can both go undetected with unsafe power levels exiting off the broken fiber at both locations.
FIG. 4, a second reflection-based embodiment uses dither in the optical signal to partially address the difficulty in using reflection-based detectors in combination with downstream Raman pumps. For example, EDFA circuit packs 38 and 52 produce light signals exhibiting dither characteristics, such that a fiber discontinuity at 56 produces a change in dither tone in an optical signal received at back-reflection optical detector 58. Back-reflection optical detectors 58 and 60 are adapted to detect LOS as a function of change dither tone, such that optical power level contributions of Raman pump 40 do not affect the ability of the back-reflection optical detectors to detect downstream LOS in the same way. However, dither tone detection-based mechanisms are not immune to the power level uncertainties caused by distance and characteristics of the fiber end-face. Further, dithering of multiple emission sources that share the same optical fiber are known to cause unwanted cross-talk (transfer of the dither tone from one optical wavelength to another). This cross-talk limitation is especially true of Raman-assisted transmission systems. Still further, the dither tone detection-based solution fails to detect a fiber discontinuity at 42.
For further understanding of telemetry-based solutions, reference may be had to the following patents: U.S. Pat. No. 5,513,029, entitled Method and Apparatus for Monitoring Performance of Optical Transmission Systems, issued to Roberts; U.S. Pat. No. 5,859,716, entitled Self-Stimulation Signal Detection in an Optical Transmission System, issued to O'Sullivan et al.; and U.S. Pat. No. 6,317,255 B1, entitled Method and Apparatus for Controlling Optical Signal Power in Response to Faults in an Optical Fiber Path, issued to Fatehi et al.
The need remains for an automatic power management solution that does not rely on non-critical channels to determine disruptions to fiber connectivity and/or trigger service-impacting decisions. The need further remains for a solution that does not rely on specific attributes of the discontinuity (connector type or location of fault) to function effectively. The present invention provides such a solution.